Signal modulators are utilized in a number of applications, for instance as part of the conversion from analogue signals to digital signals or vice versa.
Conventionally analogue-to-digital converters (ADCs) and digital-to-analogue converters (DACs) are implemented using multiple analogue components such as banks of well-matched or ratioed resistors, capacitors or current sources. However there is an ongoing trend to smaller semiconductor process node geometries and such circuitry does not scale efficiently in terms of semiconductor area and/or power requirements.
Time-encoding modulators are modulators that encode input data into a time-encoded data stream. The time-encoded data stream can then be decoded using a time decoding converter. One particular application of a time-encoding modulator is as part of an analogue-to-digital converter (ADC) where a time-encoding modulator is used to encode an input analogue signal as the duty cycle of a pulse-width modulated (PWM) signal. The PWM signal is input to a time-decoding converter (TDC) which outputs a digital signal indicative of the duty-cycle of the PWM signal. In practice the TDC has a voltage-controlled oscillator (VCO) which is controlled based on the PWM signal and a counter arranged to count the oscillations in the VCO output. The average count value over a period of time depends on the duty cycle of the PWM signal and thus can be used as digital representation of the input analogue signal.
Such an ADC circuit may be used in a number of applications. Increasingly however there is a demand for low power operation. For example, as voice control is becoming more popular for electronic devices, there may be a need for audio circuitry of the electronic device to be always-on to listen for a possible voice command. The processing to detect voice commands is implemented digitally so there may be implementations requiring an ADC circuit to continually operate to convert analogue audio signals from a microphone transducer into digital audio signals for voice detection. Similarly, there is a trend to replace mechanical control buttons by virtual buttons relying on force-sensing transducers to sense local pressure applied by a user and requiring some continual conversion of one or more analogue force sensor output signals to a digital form for convenient processing for robust detection of user input in an always-on modality. Especially for battery powered devices, power consumption is a concern and thus low power operation is often desirable.
One issue with an ADC circuit such as described above is that typically the VCO needs to operate at relatively high frequencies to provide acceptable noise performance and this can result in a relatively high power consumption. VCO circuits implemented in smaller process node geometries still suffer from relatively high power requirements.
In general therefore there is thus a desire for modulators that can be used, for example, as part of a signal converter such as an ADC that can be operated with relatively low power.